1. Field of the Invention
The present invention relates to radiation source locators, and particularly to a differential ultra-wideband (UWB) indoor positioning method that accurately determines the location of the UWB radiation source.
2. Description of the Related Art
The interest of research and industry in indoor positioning has greatly increased recently. The market of indoor positioning services include applications in, e.g., health care, search and rescue (SAR), logistics, and security. Unfortunately, the global positioning system (GPS), which performs satisfactorily in most outdoor environments, could not guarantee availability or meet the accuracy requirements for indoor use. These shortages are mainly due to complete or partial signal blockage and extreme multipath conditions.
Ultra-wideband (UWB) technology is considered a promising technology, which could meet the requirements of successful employment of indoor localization systems, since the release of the U.S. Federal Communications Commission (FCC) First Report and Order in 2002 covering commercial use of UWB. High time resolution of impulse radio-based UWB signals, their ability to penetrate through walls and obstacles, and their resistance to jamming and multipath effects are potential properties for indoor positioning capability. Moreover, low cost and low complexity are major advantages of impulse radio-based UWB systems. Localization algorithms based on the time of arrival (TOA) or time difference of arrival (TDOA) measurements yield accuracies in the centimeter level in line-of-sight (LOS) situations. These attractive characteristics are reached at high system costs due to demanding hardware requirements, i.e., very high sampling rates and sub-nanosecond level synchronization between the involved stations/sensors and the object/source to be localized.
A low cost and low complexity UWB ranging system can be constructed based on the received signal strength (RSS) measurements and taking advantage of the small fading characteristic of the UWB signals. The channel characteristics of the target environment have to be sufficiently determined a priori. However, if the source is not very close to enough sensors, the resulting accuracy is in the few meters level and deteriorates for longer distances, even in the LOS case. The major disadvantages of the angle of arrival (AOA)-based positioning technique are the additional costs associated with employment of antenna arrays and the increased computational complexity of AOA estimation due to significant multipath time dispersion of an indoor UWB signal, i.e., a very large number of paths.
The main sources of error in TOA measurements are clock synchronization errors, sensor location uncertainty, multipath propagation, and non-line-of-sight (NLOS) delays due to propagation through materials denser than air. The literature provides several techniques to partially compensate for NLOS errors, e.g., using statistical information of NLOS error, NLOS identification algorithms, or location fingerprinting. Moreover, in a multi-source environment, the accuracy of TOA measurements can be further degraded due to multiple access interference (MAI). Therefore, UWB sources can use different time slots for transmission in order to help reduce the MAI effects.
Thus, a differential ultra-wideband indoor positioning solving the aforementioned problems is desired.